The present invention relates to an electrophotosensitive material which is used in image forming apparatuses such as laser printer, electrostatic copying machine, plain paper facsimile device, combined device having these functions and the like.
In the image forming apparatuses described above, so-called organic photosensitive materials comprising an electric charge generating material which generates a charge by irradiation with light, an electric charge transferring material which transfers the generated charge, and a binder resin constituting a layer in which these substances are dispersed have widely been used. In general, these organic photosensitive materials are classified roughly into an electrophotosensitive material comprising a single-layer type photosensitive layer wherein the same layer contains an electric charge generating material and an electric charge transferring material, and an electrophotosensitive material comprising a multi-layer type photosensitive layer formed by laminating an electric charge generating layer containing an electric charge generating material and an electric charge transferring layer containing an electric charge transferring material.
Various trials of improving electrophotosensitive materials have hitherto been made and those described in U.S. Pat. No. 6,120,955, U.S. Pat. No. 5,955,230, U.S. Pat. No. 5,958,638, U.S. Pat. No. 5,942,362, U.S. Pat. No. 5,932,384, U.S. Pat. No. 5,932,722, U.S. Pat. No. 5,753,395, U.S. Pat. No. 6,045,957 and U.S. Pat. No. 6,015,646 have been known.
However, conventional electrophotosensitive materials have the following problems at present.
(1) Although the surface of a photosensitive material is charged with a positive or negative electrostatic charge after the charging step during the formation of an image, the bottom of a photosensitive layer is charged with a charge having a polarity which is reverse to the polarity of the surface of the photosensitive material. In case an intermediate layer is absent, the charge generated on the bottom of the photosensitive layer is removed via a conductive supporting substrate. Therefore, when the photosensitive material is subjected to light exposure, the charge of the surface of the photosensitive material is not transferred to the supporting substrate (earth) and remains on the surface of the photosensitive material, thereby to cause image fog.
(2) In case a photosensitive layer is directly coated on a supporting substrate, a photosensitive layer is not sufficiently bound onto the supporting substrate, sometimes, depending on the kind and coating conditions of a binder resin.
(3) In case defects such as scratch are present on the surface of a supporting substrate, black dot is formed on an image.
To solve the problems described above, a method of forming an intermediate layer containing a binder resin on a supporting substrate and forming a photosensitive layer thereon is suggested. According to this method, formation of the intermediate layer prevents a charge generated on the bottom of the photosensitive layer from removing easily and also strong binding of the photosensitive layer on the supporting substrate covers defects on the surface of the supporting substrate, thereby making it possible to smoothen the surface.
The binder resin used in the intermediate is preferably a thermosetting resin. The reason is as follows. That is, when the thermoplastic resin is used, the intermediate layer is dissolved and deteriorated in case an electric charge generating layer is formed on the intermediate layer by coating, depending on the kind of a solvent of a coating solution for electric charge generating layer, thereby making it impossible to coat the electric charge generating layer uniformly and homogeneously.
When the thermosetting resin is used as the binder resin, the intermediate layer is formed by coating a coating solution prepared by dissolving the thermosetting resin in the solvent and subjecting the coated supporting substrate to a heat treatment, thereby to cure the thermosetting binder resin.
However, in case the heat treatment is not sufficiently carried out, the curing degree of the thermosetting resin is reduced, thereby to cause the same problems as in case of the thermoplastic resin. Also since the electric conductivity is lowered, there arises such a problem that the residual potential of the photosensitive material is enhanced. As a result, the toner is developed at the non-image portion, thereby causing image fog.
Since electric characteristics of the photosensitive material can be presumed by measuring the curing degree after forming the intermediate layer, it is made possible to remove defective before forming the photosensitive layer.
As the method of measuring the curing degree of the thermosetting resin, Japanese Published Unexamined Pat. (Kokai Tokkyo Koho Hei) No. 5-19518 discloses a method for quantization of the curing degree, which comprises measuring an absorption intensity ratio of an infrared spectrum originating in an epoxy resin (thermosetting resin) based on the fact that an absorption peak of a carbonyl group in infrared absorption originating in polyester (thermoplastic resin) contained in the surface layer is nearly in a saturated state, thereby to measure a comparative amount of residual epoxy groups.
According to the method described above, in case the use of the thermoplastic resin is not required, the intermediate layer must contain the thermoplastic resin for the purpose of only measuring the curing degree. Moreover, it is troublesome because the measurement of the infrared absorption spectrum requires a long time.
Thus, an object of the present invention is to solve the technical problems described above and to provide an electrophotosensitive material (sometimes abbreviated to as a xe2x80x9cphotosensitive materialxe2x80x9d, hereinafter) capable of forming a good image, which has a low residual potential as compared with the prior art and is free from fog.
Another object of the present invention is to provide a method of producing an electrophotosensitive material, which does not forward any defective to the following step, by presuming a residual potential of a photosensitive material in the state of an intermediate during the formation of an intermediate layer.
Still another object of the present invention is to provide a method of producing an electrophotosensitive material, which causes fewer scattering in residual potential.
To solve the problems described above, the present inventors have found a factor, which has a correlation with the curing degree of the thermosetting resin and is easy to measure, and studied to determine an acceptable range of the factor by a correlation between the factor and the residual potential of the photosensitive material.
As a result, they have found that a contact angle is suited for use as the factor. It has been found that the residual potential decreases with an increase in contact angle, while a change in residual potential nearly disappears when the contact angle exceeds a certain value.
An electrophotosensitive material having a contact angle [contact angle which enables the residual potential to become stable] predetermined from a correlation between the contact angle of the intermediate layer and the residual potential of the photosensitive material on the basis of the correlation described above is produced, thus completing the present invention.
The electrophotosensitive material of the present invention comprises a supporting substrate, an intermediate layer containing a thermosetting resin formed on the supporting substrate, and a photosensitive layer formed on the intermediate layer, wherein a contact angle of the surface of the intermediate layer is not less than a value (Axc2x0) represented by the formula: Axc2x0=Bxc2x0xe2x88x922xc2x0 in which Bxc2x0 is a contact angle corresponding to an intersection of a first approximation linear line and a second approximate linear line in a correlation curve between a residual potential of the photosensitive material comprising the predetermined photosensitive layer formed on the intermediate layer containing the thermosetting resin and a contact angle of the intermediate layer containing the thermosetting resin; and the first approximate linear line denotes an approximate linear line of the portion where the residual potential decreases proportionally with an increase in contact angle in the correlation curve, while the second approximate linear line denotes an approximate linear line of the portion where a change in residual potential with an increase in contact angle nearly disappears.
The first method of producing an electrophotosensitive material of the present invention comprises forming an intermediate layer containing a thermosetting resin on a supporting substrate, measuring a contact angle of the surface of the intermediate layer, and forming a photosensitive layer on the intermediate layer when the contact angle is within a predetermined range.
The second method of producing an electrophotosensitive material of the present invention comprises forming an intermediate layer containing a thermosetting resin on a supporting substrate, carrying out a heat treatment so that a contact angle is set within a predetermined range, and forming a photosensitive layer on the intermediate layer when the contact angle is within a predetermined range.
The electrophotosensitive material of the present invention is capable of forming a good image, which has a low residual potential and is free from fog. According to the first method of producing an electrophotosensitive material of the present invention, since a residual potential of a photosensitive material can be presumed in the state of an intermediate during the formation of an intermediate layer, it does not forward any defective to the following step. Also according to the second method of producing an electrophotosensitive material of the present invention, since an intermediate layer is formed under heat treatment conditions which reduce scattering in residual potential, it is made possible to stabilize the quality.